One of the main contributors to the accumulation green-house gases is the petroleum based energy sources, for which it has been predicted that for the next 50 years will add more than 400 gigatonnes of carbon dioxide into the atmosphere. A reasonable approach to solve this quandary may be the use of natural gas. However, significant amounts of hydrogen sulfide, nitrogen and carbon dioxide could make its efficiency as an energy source lower than that of petroleum based fuels. The removal of these species from natural gas effluents makes it also a less cost effective alternative, since the most common purification methods involve physical adsorption of CO2 with a solvent and this requires a substantial energy input during the regeneration stage. Other alternatives include cryogenic methods, which could achieve acceptable removal efficiencies but are also energy intensive. The selective removal of CO2 via adsorption processes at or near ambient conditions, on the other hand, could be an attractive solution from the energy consumption point of view, yet many of the available adsorbent materials still posses low working capacities. This problem, however, could be solved via implementation of bottom-up synthesis strategies in an attempt to produce adsorbents with framework properties that permit larger saturation capacities while sustaining the selectivity features. Furthermore, these characteristics could make the said materials suitable for closed-volume applications in which atmospheric control or revitalization is required, such as in spacecraft cabins in which breathable air require ultra-low carbon dioxide concentrations. The main challenge is to find inorganic compositions that will permit an increase in micropore surface area while allowing surface tailoring and modulation of the dimensions of the pore entrance.
Titanosilicates with mixed octahedral-tetrahedral units may provide the necessary requirements to produce adsorbents for the deep removal of CO2. The combination of titanium centers with multiple coordination states allows the structure to be flexible upon dehydration while allowing for cation exchange (i.e., effective surface functionalization). In addition, the pore channel geometry and dimensions of these matrices can be controlled by employing a template or structure directing agent (SDA) during synthesis. Such is the case of a material known as UPRM-5, synthesized by Hernández and co-workers by using tetraethylammonium (TEA+) cations as the SDA and which differs from other flexible titanosilicates (e.g., ETS-4). The use of a template during the synthesis of UPRM-5 resulted in a material with an enhanced thermal stability range and larger adsorption capacity, still without compromising the thermal pore contraction property. However, there is still much more to learn about how the type and nature of the SDA controls the coordination of the titanium centers and the level of structural faulting that gives origin to the thermal flexibility of the framework. Knowledge of this information would permit the design of more robust adsorbents to address the great challenge of reducing carbon dioxide emissions.